The present invention relates to a copier, facsimile apparatus or similar image forming apparatus and, more particularly, to an image forming apparatus having book reading and page turning capabilities and implementing high-speed image formation on both sides of a sheet.
An automatic document feeder (ADF) is extensively used with an image forming apparatus of the kind described as an image reading device. The ADF automatically transports a sheet document to a reading position, reads the image of the document, and then discharges the document from the reading device. So far as the document is a sheet document, the ADF can read it automatically. However, regarding a book document, it is extremely difficult to automate the page turning operation of the ADF. The only measure left at the present stage of development is to turn the pages of a book manually. Although various methods and means have, of course, been proposed for reading a book document automatically, most of them are merely conceptual and far from practicality.
In the light of this, Japanese Patent Laid-Open Publication (Kokai) No. 2-193589 discloses a device capable of reading a book document while turning the pages thereof with a page turning and reading unit, or scanning unit as referred to hereinafter. The scanning unit has thereinside page receiving means, page attracting means, page separating means, reading means, etc. Specifically, the scanning unit has a page turning belt extending along the surface of a document table. While a document is laid on the document table in a spread position between the surface of the table and the belt, the scanning unit is moved relative to the document while causing the belt to form a roundabout portion moving away from the table. With this kind of device, it is possible to fully automate the page turning and image reading operation which has heretofore been time- and labor consuming and, therefore, to implement a multifunction image reading system remarkably enhancing the productivity of, for example, a copier.
When the device having book reading and page turning capabilities as stated above is used as a scanner of an image forming apparatus operable in a duplex mode for forming images on both sides of a sheet or similar medium, there can be implemented an image forming apparatus easy to operate when a book is to be read. However, an image forming apparatus operable in a duplex mode is usually constructed such that image data read out of documents are sequentially formed on the front and rear of a sheet in the same order as they have been read. Further, it has been customary with this kind of apparatus to form an image on the front of a sheet, turn over the sheet, refeed the sheet to an image forming section by a refeed mechanism, and then form an image on the rear of the sheet. Therefore, when images representative of two pages of a spread book are reproduced on the front and rear of a sheet in the same order as they have been read, the pages of the sheet and those of the book do not correspond to each other, resulting in a copy different from the original. Moreover, since a sheet carrying an image on the front thereof has to be turned over and then refed by the refeed mechanism before the formation of an image on the rear thereof, the image forming speed in the duplex mode is extremely low.